Moreover, since he frequently calls the Christian doctrine a secret system (of belief), we must confute him on this point also, since almost the entire world is better acquainted with what Christians preach than with the favourite opinions of philosophers. For who is ignorant of the statement that Jesus was born of a virgin, and that He was crucified, and that His resurrection is an article of faith among many, and that a general judgment is announced to come, in which the wicked are to be punished according to their deserts, and the righteous to be duly rewarded?
And yet the mystery of the resurrection, not being understood, is made a subject of ridicule among unbelievers. In these circumstances, to speak of the Christian doctrine as a secret system, is altogether absurd. But that there should be certain doctrines, not made known to the multitude, which are (revealed) after the exoteric ones have been taught, is not a peculiarity of Christianity alone, but also of philosophic systems, in which certain truths are exoteric and others esoteric.
Some of the hearers of Pythagoras were content with his ipse dixit; while others were taught in secret those doctrines which were not deemed fit to be communicated to profane and insufficiently prepared ears. Moreover, all the mysteries that are celebrated everywhere throughout Greece and barbarous countries, although held in secret, have no discredit thrown upon them, so that it is in vain that he endeavours to calumniate the secret doctrines of Christianity, seeing he does not correctly understand its nature.
Source: Against Celsus (New Advent)